Did the plants that died show any specific symptoms before they died, such as spots on the leaves, yellowing leaves, cracking or oozing trunks/stems? These could point to a disease problem or a nutrient deficiency in the site's soil. Another possibility is that salt or other toxic substances from a nearby road, or herbicides from a neighbor, could have entered the soil and killed the plants. This is especially a possibility if the site is very close to the road: